Hydrostatic transmissions are well known and generally may include a hydraulic pump and a hydraulic motor. The hydraulic pump and the hydraulic motor may be arranged as separate components or may be combined together in an integral unit.
One use for such hydrostatic transmissions is, for example, zero turn radius vehicles such as zero turn radius lawn mowers. A separate hydrostatic transmission hydraulic pump and motor combination on each lateral side of the vehicle may drive one of the drive wheels of the vehicle. When the output hydraulic fluid flow rate of the two pumps is equal and the hydraulic motors rotate their associated wheels of the vehicle in the same direction at the same speed, the vehicle travels in a substantially straight line path in either the forward or the reverse direction. When the output hydraulic fluid flow rate of the two pumps is not equal and the hydraulic motors rotate their associated wheels in the same direction but at different speeds, the vehicle will turn. When one of the pump and motor combinations is rotating its associated wheel in one direction and the other pump and motor combination is rotating its associated wheel in the other direction at the same rotational speed, the vehicle will make a zero radius turn. An operator interface allows the vehicle operator to control the output hydraulic fluid flow rate of the two pumps to the motors, to control straight line or turning or zero radius turns for the vehicle.
In hydrostatic transmissions and systems and methods and vehicles, a prime mover engine such as for example a gasoline or diesel fuel engine frequently has a substantially vertical output shaft. The prime mover substantially vertical output shaft may drive a substantially vertical hydraulic pump input shaft through a drive mechanism such as for example a flexible drive belt. Because the drive wheels and axles of the vehicle necessarily have a substantially horizontal axis, the hydraulic motor output shaft of such hydrostatic transmissions that drives the axles and wheels may usually be arranged along a substantially horizontal axis. This arrangement of the hydraulic pump along a substantially vertical axis and the hydraulic motor along a substantially horizontal axis may require a substantially ninety degree mechanical and hydraulic interface connection between the pump and the motor. The ninety degree interface connection mechanically connects the pump and motor and hydraulically connects substantially vertically facing hydraulic fluid ports of the pump with substantially horizontally facing hydraulic fluid ports of the motor. The ninety degree interface connection may be relatively complex to manufacture.